Her-2/neu overexpression induces NF-κB via a PI3-kinase/Akt pathway involving calpain-mediated degradation of Iκ-B-α that can be inhibited by the tumor suppressor PTEN

The Nuclear Factor (NF)-kappaB family of transcription factors controls expression of genes which promote cell growth, survival, and neoplastic transformation, Recently we demonstrated aberrant constitutive activation of NF-kappaB in primary human and rat breast cancer specimens and in cell lines. Overexpression of the epidermal growth factor receptor (EGFR) family member Her-2/neu, seen in approximately 30% of breast cancers, is associated with poor prognosis. Previously, Her-2/neu has been shown to signal via a phosphatidylinositol 3 (PI3)-kinase to Akt/protein kinase B (PKB) pathway. Since this signaling pathway was recently shown to activate NF-kappaB, here we have tested the hypothesis that Her-2/neu can activate TF-kappaB in breast cancer, Overexpression of Her-2/neu and EGFR-4 in Ba/F3 cells led to constitutive PI3- and Akt kinase activities, and induction of classical NF-kappaB (p50/p65). Similarly, a tumor cell line and tumors derived from MMTV-Her-2/neu transgenic mice displayed elevated levels of classical NF-kappaB, Engagement of Her-2/neu receptor downregulated the level of NF-kappaB. NF-kappaB binding and activity in the cultured cells was reduced upon inhibition of the PI3- to Akt kinase signaling pathway via ectopic expression of kinase inactive mutants, incubation with wortmannin, or expression of the tumor suppressor phosphatase PTEN, Inhibitors of calpain, but not the proteasome, blocked I kappaB-alpha degradation. Inhibition of Akt did not affect IKK activity, These results indicate that Her-2/neu activates NF-kappaB via a P13- to Akt kinase signaling pathway that can be inhibited via the tumor suppressor PTEN, and is mediated by calpain rather than the I kappaB kinase complex.